Detection of complete dihydropyrimidine dehydrogenase deficiency in a Tunisian family using a simple phenotypic test.

Sir,

Dihydropyrimidine dehydrogenase (DPD) is the first rate-limiting enzyme in the catabolic pathway of the anticancer agent 5-fluorouracil (5-FU) but also of the endogenous uracil (U) and thymine (TH). Administration of standard doses of 5-FU to cancer patients with a partial or complete deficiency of DPD activity has been associated with severe and sometimes life-threatening toxicities. Thus, screening for DPD deficiency could be very useful, to prevent severe and fatal toxicities in cancer patients treated with 5-FU.

In our previous study, a cancer patient was diagnosed to have profound DPD deficiency.[1] The phenotypic profile showed undetectable plasmatic 5,6 dihydrouracil (UH2) and a high accumulation of plasmatic U.[1] In the present study, we propose a high-performance liquid chromatography (HPLC)-based method as a phenotypic test used for the pre-therapeutic assessment of complete DPD deficiency in newly diagnosed cancer patients. Five healthy family members of a previously diagnosed cancer patient were investigated in order to establish a DPD-deficient phenotypic profile as an index of complete DPD deficiency. The results showed the same chromatographic profile as the one observed in the patient, with no detectable UH2 [Figure 1a] and a very high accumulation of plasmatic U (between 2307 and 3210 ng / mL versus 50 to 90 ng / mL in controls[2]), found in three subjects. Two other family members showed a normal chromatographic profile [Figure 1b]. These findings suggested that a chromatographic profile with undetectable UH2 and very high-level of plasmatic U could be used as a predictive marker of a complete DPD deficiency phenotype. In this light, Johnson et al., reported a profound DPD deficiency case with absent DPD activity and a particularly elevated plasmatic U levels.[3] The result obtained by Johnson et al., was completely consistent with our findings. Moreover, another study showed that the Uracil / dihydrouracil plasmatic ratio could be used as a phenotypic test for the assessment of DPD deficiency,[4] which was also in agreement with our findings.

Figure 1

(a) chromatogram obtained from the plasma extract of the patient's mother :undetectable UH2 was observed and a very high accumulation of U was detected suggesting that the subject was profoundly DPD deficient (direct ultraviolet detection at 205 nm) (b) chromatogram obtained from the plasma extract of the patient's father. The two metabolites were detected (direct ultraviolet detection at 205 nm). The UH2 peak was higher than the U peak suggesting a normal UH2/U plasmatic ratio

The molecular analysis performed on the patient's DPD gene (DPYD) revealed the absence of known pathogenic mutations.[1] Therefore, we looked for some mutations that could be responsible for the deleterious effects on the DPD activity in the studied subjects, but none of these mutations was found. This data demonstrated that the absence of known mutations in some cancer patients and the great number of mutations involved in DPD deficiency could be a limitation for the molecular approaches used as tools to screen for DPD deficiency in cancer patients. The simple and rapid phenotypic approach described in this report could be a reliable method to pre-screen DPD deficiency in cancer patients.